1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices used for resonators or band-pass filters and a method for manufacturing the same. More specifically, the present invention relates to a SAW device including an insulating layer that covers an interdigital transducer (IDT) electrode.
2. Description of the Related Art
In duplexers (DPX) or radio frequency (RF) filters used in mobile communication systems, a wideband characteristic and a favorable temperature characteristic must be satisfied at the same time. SAW devices that have conventionally been used for DPXs or RF filters use a piezoelectric substrate made of 36° to 50°-rotated Y-cut X-propagating LiTaO3. This piezoelectric substrate has a temperature coefficient of frequency of about −40 ppm/° C. to about −30 ppm/° C. As a method for improving the temperature characteristic, a method for forming a SiO2 film having a positive temperature coefficient of frequency to cover an IDT electrode on a piezoelectric substrate is known. FIG. 30 shows an example of a method for manufacturing this type of SAW device.
As shown in FIG. 30A, a resist pattern 52 is formed on a piezoelectric substrate 51 except in a region where an IDT electrode is to be formed. Then, as shown in FIG. 30B, an electrode film 53 defining an IDT electrode is formed over an entire surface. Then, the resist pattern 52 and the metal film on the resist pattern 52 are removed using a resist removing agent. In this manner, an IDT electrode 53A is formed, as shown in FIG. 30C. Then, as shown in FIG. 30D, a SiO2 film 54 is formed to cover the IDT electrode 53A.
Japanese Unexamined Patent Application Publication No. 11-186866 (Patent Document 1) discloses a method for manufacturing a SAW device including an insulating or semiconducting protective film covering an IDT electrode of the SAW device, the method is used for reasons other than improving the above-described temperature coefficient of frequency. FIG. 31 is a schematic cross-sectional view showing the SAW device described in this known art. In the SAW device 61, an IDT electrode 64 made of Al or an alloy primarily including Al is disposed on a piezoelectric substrate 62. An insulating or semiconducting inter-electrode-finger film 64 is disposed in a region except the region where the IDT electrode 63 is disposed.
Furthermore, an insulating or semiconducting protective film 65 is arranged to cover the IDT electrode 63 and the inter-electrode-finger film 64. In the SAW device 61 described in this known art, the inter-electrode-finger film 64 and the protective film 65 are made of an insulating material, such as SiO2, or a semiconducting material, such as silicon. According to this known art, forming the inter-electrode-finger film 63 suppresses degradation of a characteristic caused by inter-electrode-finger discharge due to a pyroelectric property of the piezoelectric substrate 61.
On the other hand, Japanese Unexamined Patent Application Publication No. 61-136312 (Patent Document 2) discloses a one-port SAW resonator. This one-port SAW resonator is manufactured by forming an electrode made of metal, such as aluminum or gold, on a piezoelectric substrate made of quartz or lithium niobate, forming a SiO2 film thereon, and then smoothing the SiO2 film. According to Patent Document 2, a favorable resonance characteristic is obtained by smoothing.
As shown in FIGS. 30A-30D, in the known method for manufacturing a SAW device of forming a SiO2 film, in order to improve a temperature coefficient of frequency, the height of the SiO2 film 54 differs at a portion where the IDT electrode 53A exists and at a portion where the IDT electrode 53A does not exist. Such an uneven surface of the SiO2 film 54 disadvantageously increases insertion loss. This unevenness becomes more significant as the thickness of the IDT electrode increases. Therefore, the film thickness of the IDT electrode cannot be increased.
In the SAW device described in Patent Document 1, the inter-electrode-finger film 64 is formed between electrode fingers of the IDT electrode 63 and then the protective film 65 is formed thereon. In this method, the surface of the protective film 65 can be smoothed.
However, in the structure described in Patent Document 1, the IDT electrode 63 is made of Al or an alloy primarily including Al. The inter-electrode-finger film 64 contacts the IDT electrode 63, however, a sufficient reflection coefficient cannot be obtained in the IDT electrode 63. Accordingly, ripples occur in a resonance characteristic, for example.
In the manufacturing method described in Patent Document 1, a resist formed on the inter-electrode-finger film 64 must be removed using a resist removing agent before forming the protective film 65. At this time, however, the IDT electrode 63 may be corroded by the resist removing agent. For this reason, a metal that is susceptible to corrosion cannot be used as a material of the IDT electrode. In other words, the type of metallic material of the IDT electrode is limited.
On the other hand, in the one-port SAW resonator described in Patent Document 2, a specific preferred embodiment includes only an example in which an electrode made of Al is formed on a quartz substrate, although it is described that quartz or lithium niobate is used for the piezoelectric substrate and that the electrode is made of aluminum or gold. That is, Patent Document 2 does not refer to a SAW device using another substrate material or another metallic material.